17 R4, black screen

I have an Alienware 17 R4 laptop that has no display on the internal screen. Plugging it into an external monitor however shows the display. I have tried disabling the Intel display, but this has made no difference. I have also downloaded and installed the latest NVidia drivers which has made no change. If I extend the display from the monitor across to the laptop display and then drag a window across, I can see a faint outline of the window as if the backlight is blown (to use an old LCD term). I thought it may be brightness, but turning it up produces no change. Does anyone have any suggestions?